I just purchased a used 2007 Mallard 27' tt. I was going through it to make sure the lights and appliances all worked when I came across a problem. In my slide out the 12vdc interior lights do not come on. There is power to the switch and when I take out the light bulbs there is power at the light. Where I am confused is when I put a brand new bulb in the light and test power again at the light and switch there is no 12v anymore. I dont understand how there could be power before the light bulb then none after a bulb is installed. Please help!!! FYI the other interior lights and all 12vdc appliances all work with no problems and batteries do charge.

My second problem is at the distribution panel when I tested all my fuses (all good) I noticed that when they were removed there was 12vdc present on both sides of where the fuse goes. If there is power on both sides what is the point of having a fuse in there since the power doesnt seem to go through it? Is this normal on travel trailers because once a fuse in a car is removed then there is no power on the other side. Please help!!!

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Sounds like a dead short, you have power until you add a load on the line. Not the same but I had this at the campground whereI work on a pedistal.
We had full power until a load was added, I pluged in a hair dryer and we had 25V blead out on the water faucet. You may have a bare wire under the slide shorting on the frame.

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Scott and Jeanna, 2000 Sunnybrook 29DBS TT
Sorry we missed church, we were camping in the woods practicing witchcraft.

With a short would the fuse still be good? I put the slide out and looked at the wire that extend with the slide but I didnt see any exposed wires. While I was down there i did wrap elec tape around any wire that was not in the plastic tubing. I guess I will have to take a real close look at where ever those wires go throughout the trailer. Thanks

Your ground buss is not ground. Test the buss to frame. Frame might also not be ground. Measure you voltage between the frame and the ground. If that is ground then look for a loose connect between buss and frame.

As Mentioned,Bad Ground #1, Bad connection in the light fixture #2, Defective connection at the switch or bad switch.
If you are reading 12 volts with a digital Multimeter the circuit load is so low you can read potential voltage of the circuit.
if all the above items check out,and the fuse is good. Check for a loose or broken wire. Good test is to use 12 volt pigtail and connect it to the positive wire and ground the other one somewhere.
good light =s good 12 volt feed. then t has to be the ground or light fixture.